There should have been 56 crews operating, instead of the 44 who were on duty.

The Northern Ireland Ambulance service (NIAS) confirmed it operated at a reduced level of cover on Saturday night.

The shortage of 24 staff meant delays for "less serious calls", and was due to a number of factors including rostered leave, sickness, vacant shifts and cancelled overtime. NIAS had said it would seek support from voluntary and private ambulance crews.

Last night NIAS was five crews short, a spokesman said.

UUP health spokeswoman Jo-Anne Dobson said the shortage was "completely and wholly unacceptable".

"I am deeply concerned that the dedicated staff within the Ambulance Service are being pushed beyond breaking point, which is a major contributing factor to these staff shortages and the lengthening response times identified earlier this year," she said. "With so many Ambulance Service personnel now opting to work part-time hours, it raises serious questions about the sustainability of the service.

"It is deeply concerning to hear from the Ambulance Service themselves that reductions and shortages in the order of 12 crews has become, in their own words, 'pretty standard'."

South Down MP Margaret Ritchie urged the Executive to take action.

"This decision which the Northern Ireland Ambulance Trust was forced to take due to a lack of sufficient staff resources could have placed some communities in danger," she said.

"This is a clear warning to the Ministers for Health and Finance that adequate and meaningful monies must be allocated to the Northern Ireland Ambulance Trust so they can recruit sufficient paramedics to undertake an accessible local health and medical service that is able to deal with emergency calls for those who are sick - particularly in rural communities like South Down," she added.

John McPoland from NIAS said it was an "ongoing issue" affecting patients with conditions "not immediately or potentially life threatening". "The trust seeks to mitigate against this by supplementing our emergency cover with support crews from voluntary and private ambulance companies to respond to the less serious calls received in our control centre. This will enable NIAS crews to respond to the life-threatening and more serious calls."

He anticipated eight external crews would be provided, adding: "We will seek to respond to all calls received but delays may be experienced for those less serious calls."